Bonding and Reactivity of a Pair of Neutral and Cationic Heterobimetallic RuZn2 Complexes

A combined experimental and computational study of the structure and reactivity of two [RuZn2Me2] complexes, neutral [Ru(PPh3)(Ph2PC6H4)2(ZnMe)2] (2) and cationic [Ru(PPh3)2(Ph2PC6H4)(ZnMe)2][BArF4] ([BArF4] = [B{3,5-(CF3)2C6H3}4]) (3), is presented. Structural and computational analyses indicate these complexes are best formulated as containing discrete ZnMe ligands in which direct Ru–Zn bonding is complemented by weaker Zn···Zn interactions. The latter are stronger in 2, and both complexes exhibit an additional Zn···Caryl interaction with a cyclometalated phosphine ligand, this being stronger in 3. Both 2 and 3 show diverse reactivity under thermolysis and with Lewis bases (PnBu3, PCy3, and IMes). With 3, all three Lewis bases result in the loss of [ZnMe]+. In contrast, 2 undergoes PPh3 substitution with PnBu3, but with IMes, loss of ZnMe2 occurs to form [Ru(PPh3)(C6H4PPh2)(C6H4PPhC6H4Zn(IMes))H] (7). The reaction of 3 with H2 affords the cationic trihydride complex [Ru(PPh3)2(ZnMe)2(H)3][BArF4] (12). Computational analyses indicate that both 12 and 7 feature bridging hydrides that are biased toward Ru over Zn.


■ INTRODUCTION
Heterobimetallic complexes comprised of a transition metal (TM) and a main group metal (MGM) are the focus of considerable interest 1−3 because of the possibility that the disparate chemistry of the two partners could combine cooperatively to bring about the novel stoichiometric and/or catalytic activation of small molecules. 4−9 In one recent example, shown in Scheme 1a, the challenging C−O activation of an anisole takes place across the Rh−Al bond of complex I to afford II, which upon addition of a silane, mediates catalytic C−O bond reduction. 10 Complex I represents one class of heterobimetallic complexes in which the MGM forms part of a multidentate ligand on the TM center. 11 Another class of complex is represented by III in Scheme 1b, in which the MGM is unsupported and unconstrained. In this particular case, both Ru and Zn centers are coordinatively unsaturated, and this "dual unsaturation" allows them to act cooperatively in the stoichiometric activation of H 2 to give IV. 12 We have interpreted Ru−Zn bonding within complex III and other related Ru−Zn complexes 13,14 in terms of a donor−acceptor interaction between a Ru(0) metal center and Z-type Zn-based acceptor ligands.
Complex III is formed upon elimination of an alkane, 15−18 an approach we have used to prepare other Ru and mono-Zncontaining products, including complex 1 in Scheme 2 that features bridging hydride and aryl ligands. 12−14,19−21 Accordingly, the reaction of complex 1 with ZnMe 2 resulted in further alkane elimination and formation of neutral [RuZn 2 Me 2 ] complex 2. 14 Alternatively, reaction with a source of [ZnMe] + induced C−H reductive coupling in 1 and formation of cationic [RuZn 2 Me 2 ] complex 3. 13 Another strategy for the preparation of [TM-Zn 2 R 2 ] species 23−25 involves addition of Carmona's Cp*Zn−ZnCp* dimer to low-valent precursors. 26,27 On the basis of the isolobal nature of Cp*Zn and a hydrogen atom, the coordination of the TM center to an intact Zn−Zn bond can be considered to form an all-metal analogue of a TM(η 2 -H 2 ) complex. Likewise, weakening of the Zn−Zn interaction to the point where it gives two ZnCp* ligands has been compared to the oxidative cleavage of the η 2 -H 2 ligand to form two M−H bonds, although such Zn−Zn bond cleavage is proposed to proceed without any change in the formal oxidation state. 23,26 In such cases, ZnCp* and related ZnR (R = alkyl or aryl) ligands have been formulated as monovalent one-electron donors. 26 Computational studies have suggested that the extent to which the Zn−Zn interaction in Cp*Zn−ZnCp* is retained upon approach to a TM is dependent on the nature of the metal itself, the surrounding ancillary ligands, and the ZnR substituents. 26−28 In this context, the availability of the closely related neutral and cationic [RuZn 2 R 2 ] complexes, 2 and 3, respectively, provides an opportunity to explore the analogy between {RZn-ZnR} and H 2 . Herein, we report computational and experimental studies to this end.

■ RESULTS AND DISCUSSION
Structure and Bonding in 2 and 3. Figure 1 shows the geometries and labeling system used in the discussion of 2 and 3. In general, good agreement was seen between the experimental and fully optimized structures; however, the Zn1···C1 distances were overestimated by 0.06−0.20 Å depending on the functional used (Supporting Information). Therefore, to analyze the observed geometries, we have taken the heavy atom (i.e., non-H) positions from the crystallographic studies and optimized the H atom positions with the BP86 functional. This approach also allows for a consistent treatment of the new hydride-containing structures that we describe below, where the H atom location is intrinsically less precise.
Both 2 and 3 feature triangular RuZn 2 moieties with Ru−Zn distances that are shorter than Zn−Zn distances. The Ru−Zn distances are more symmetrical in 2 (2.50/2.51 Å) than in 3 (2.41/2.55 Å) and are all within the sum of the covalent radii ( 2 complexes, but that 2 is slightly displaced along the continuum toward a Ru(η 2 -RZn−ZnR) species. 26 Note that in 3, the Ru−Zn1−Me angle is smaller than expected at 161.3°, but in this case, the Me group is bent away from C1, suggesting that it is the short Zn1···C1 contact of 2.15 Å that drives this distortion. This is explored further in the electronic structure analyses below. Figure 2a provides details of quantum theory of atoms in molecules (QTAIM) analyses of 2 and 3 with electron density contours plotted in the {RuZn1Zn2} plane. These are complemented by noncovalent interaction (NCI) plots shown in Figure 2b. For 2, bond paths between Ru and both Zn centers are consistent with the presence of Ru−Zn bonds. The associated bond critical points (BCPs) show similar electron densities, ρ(r), of ∼0.06 au, and this relatively low value, coupled with positive values of the Laplacian and small negative total energy densities ( Figures S39 and S40), is consistent with a donor−acceptor (i.e., Ru → Zn) interaction between two heavy atoms. 30,31 Figure 2a also shows the computed delocalization indices (DI) in parentheses. These reflect the degree of shared electron density between two atomic centers 32,33 and proved more discriminating than ρ(r) for the Ru−Zn interactions. Thus, a larger DI of 0.68 is associated with the shorter Ru−Zn2 bond compared to a DI of 0.52 for the Ru−Zn1 bond. DIs can also be measured between atoms not linked by a bond path and can be useful for identifying interactions in areas of flat electron density. 34 35 A natural orbitals for chemical valence (NOCV) analysis confirmed the differences in the additional stabilizing interactions at the {Zn1Me} + fragments in 2 and 3 ( Figure  3). In each case, the key deformation density channel is dominated by donation from the dsp hybrid HOMO of the Rubased fragment into the σ* LUMO of {ZnMe} + . For 2, this also shows contributions from both Zn2 and C1, whereas for 3, a larger component from C1 is apparent but no contribution from Zn2 is seen. Equivalent plots for the {Zn2Me} + fragments are provided in Figures S46 and S47.
Reactivity Studies. Given that 2 and 3 result from the formal introduction of ZnMe and [ZnMe] + , respectively, into 1, a series of reactivity studies were undertaken to probe the potential to reverse this process, through either thermolysis or   20 Alongside formal elimination of ZnMe, 36 the formation of 5 also requires C−H/P−C activation and C−C coupling steps to generate the dppbz and metalated Ph 2 P-(biphenyl) ligand, although the exact sequence in which these steps take place remains unknown. 37 For 3, the result of heating proved to be much less clear. 31 P NMR monitoring of a reaction mixture refluxed in benzene (2 days) or refluxed in toluene (2 h) revealed formation of an initial product 9 (characterized by two coupled doublet resonances at δ 78 and 48) that, upon further heating (2 days) in toluene, converted to a second product, 10, which showed a similar pair of coupled signals at δ 53 and 46. Both compounds gave oils in all tested combinations of solvents, 38 which, together with an absence of any diagnostic (i.e., non-aromatic) 1 H NMR signals, makes their identities hard to determine.
Reactivity of 2 and 3 with Lewis Bases. Rather than removing either of the ZnMe ligands, the reaction of 2 with P n Bu 3 at room temperature led to substitution of the PPh 3 ligand and formation of [Ru(P n Bu 3 )(C 6 H 4 PPh 2 ) 2 (ZnMe) 2 ] (6). X-ray characterization ( Figure 4) revealed a structure that was broadly similar to that of 2 in terms of metrics (Table S2). The ease of phosphine substitution in 2 contrasts with the difficulties reported by Fischer in attempting to exchange phosphine ligands in multi-Zn-containing Ni species. 39 At the same time, the lack of reaction between 2 and PCy 3 indicates how sensitive these systems are to the choice of Lewis base. In contrast, 3 reacted with both P n Bu 3 and PCy 3 , and in this case, this did incur the loss of [ZnMe] + to give 1, together with 11 in the case of P n Bu 3 . 40,41 The findings fit with the previously observed complete conversion of 3 into 1 that is seen in THF. 13 The fate of the eliminated [ZnMe] + could not be established, but when the Lewis base is changed to the Nheterocyclic carbene IMes, 42 trapping as the NHC adduct [(IMes) 2 ZnMe] + was found, 43 alongside formation of 1.
A very different outcome was found when IMes was reacted w i t h 2 . This aff o r d e d [ R u ( P P h 3 ) ( C 6 H 4 P P h 2 ) -(C 6 H 4 PPhC 6 H 4 Zn(IMes))H] (7) through substitution of a  Redissolving a crystalline sample of the compound gave NMR signals for 7 together with a second, minor species, 8. 46 The signals for 7 were consistent with the solid state structure, a doublet of doublet of doublets Ru−H−Zn resonance in the 1 H NMR spectrum, with one large (pseudotrans) and two smaller 2 J HP splittings, and one high-frequency 31 P triplet (δ 52) for the PPh 3 group, together with two lower-frequency (δ −12 and −24) doublet of doublet signals for the two cyclometalated phosphines. 47,48 The presence of similar 31 P chemical shifts for the minor species 8 supports it being an isomer. While 1 H{ 31 P} NMR measurements showed that both isomers feature hydride trans to a metalated phosphine, the presence of two surprisingly small 2 J HP couplings (9 and 6 Hz), in addition to a large, pseudotrans splitting (46 Hz) in the 31 Pcoupled 1 H NMR spectrum, leaves it unclear as to exactly what the structure of 8 is. Closer inspection of NMR spectra recorded shortly after combining IMes and 2 indicates that 8 is formed in the initial stages (mixing for <15 min) and is thus a kinetic product of the reaction formed prior to subsequent growth of the thermodynamic product, 7. 49 The signals of 8 seen in the NMR spectra of 7 may therefore arise due to cocrystallization.
Reactivity of 2 and 3 toward H 2 . During our previous studies of Ru mono-Zn complexes, 12,13,19,20 H 2 was typically found to add across the Ru−Zn bond, as shown in Scheme 1b. Very different, contrasting behavior was seen with 2 and 3. Thus, the former did not react with H 2 at room temperature and, upon being heated to 60°C, gave only a complex mixture of products. In contrast, 3 reacted rapidly with two molecules of H 2 at room temperature to reverse the phosphine cyclometalation and form the cationic dizinc trihydride complex, [ Complex 12 displayed high-frequency doublet and triplet 31 P NMR resonances, consistent with the mer-RuP 3 geometry apparent in the X-ray crystal structure ( Figure 6). The 1 H NMR spectrum showed two hydride signals at δ −7.3 (dtd) and −11.1 (dtt) in a 2:1 ratio. 50 Upon being heated to 60°C, 12 decomposed as evidenced by the precipitation of an insoluble red oil at the bottom of reaction solutions.
The molecular structure of the cation in 12 is shown in Figure 6. The equatorial positions comprised two ZnMe ligands, one PPh 3 ligand, and three hydrides (which were located and refined without restraints). The coordination sphere was completed by two phosphines in a distorted trans-

■ CONCLUSIONS
A combined computational and experimental study has been undertaken on two [RuZn 2 Me 2 ] species, neutral 2 and cationic 3. Geometrical considerations supported by computational analyses confirm the presence of direct Ru−Zn bonds in both species and suggest these are best formulated as Ru(ZnMe) 2 complexes featuring discrete ZnMe ligands. Some additional stabilization may be achieved via Zn···Zn interactions, and 2 and 3 both exhibit Zn···C aryl interactions, with these being more significant in 3.
Experimentally, the two complexes exhibit diverse reactivities with thermolysis and the addition of a range of Lewis bases bringing about different outcomes with no apparent correlation to either the overall charges of the complexes or the different strengths of the Ru−Zn interactions present. 2 reacted with H 2 to give a mixture of products, while in contrast, reaction of H 2 with 3 led cleanly to   12 adds to the range of transition metal complexes that feature multiple main group metals and multiple hydride ligands that have recently attracted a great deal of attention due to the unusual bonding interactions and unusual geometries they can possess. 54,55 Studies of their reactivity, however, remain rare. 56 In the study presented here, we have shown that both the TM and the MGM can be centers of reactivity in these heterobimetallic complexes and the factors that govern the site of reactivity will be the subject of future reports from our groups. Inorganic Chemistry pubs.acs.org/IC Article ■ EXPERIMENTAL SECTION General Comments. All manipulations were carried out under argon using standard Schlenk, high-vacuum, and glovebox techniques using dry and degassed solvents. C 6 D 6 and THF-d 8 were vacuum transferred from potassium. NMR spectra were recorded at 298 K (unless otherwise stated) on Bruker Avance 400 and 500 MHz NMR spectrometers and referenced as follows: C 6 D 6 ( 1 H, δ 7.16; 13 C, δ 128.0), THF-d 8 ( 1 H, δ 1.72; 13 C, δ 25.3), and toluene-d 8 ( 1 H, δ 2.09). 31 P spectra were referenced externally to 85% H 3 PO 4 (δ 0.0). Elemental analyses were performed by Elemental Microanalysis Ltd. (Okehampton, Devon, U.K.). Compounds 1, 13 2, 14 3, 13 and IMes 57 were prepared according to literature methods.
[Ru(P n Bu 3 )(C 6 H 4 PPh 2 ) 2 (ZnMe) 2 ] (6). C 6 D 6 (0.5 mL) was added to a mixture of 2 (40 mg, 0.038 mmol) and P n Bu 3 (9.5 μL, 0.038 mM) in a J. Young's resealable NMR tube. After 15 h at room temperature, the volatiles were removed and the resulting solid was recrystallized from benzene/hexane. The microcrystalline solid was washed with hexane and dried under vacuum to give 6 as a yellow solid (18 mg, 48%). 1  [Ru(PPh 3 )(C 6 H 4 PPh 2 )(PPh(C 6 H 4 ) 2 Zn(IMes))H] (7). A mixture of 2 (40 mg, 0.037 mmol) and IMes (23 mg, 0.074 mmol) was added to a J. Young's resealable NMR tube. Addition of C 6 D 6 (0.5 mL) led to an instantaneous change in color from red to orange-yellow. NMR spectroscopy revealed that consumption of the starting material took place over 3 h to give 7 as the main product. Removal of the volatiles under reduced pressure and recrystallization of the residue from benzene/hexane gave yellow crystals of 7, which were washed with hexane and dried under vacuum (33 mg, 70% yield). 1 Figure S31)].
Thermal Decomposition of 2. A C 6 D 5 CD 3 solution (0.5 mL) of 2 (10 mg, 0.01 mmol) was heated at 80°C for 50 h, affording a dark red-brown colored solution. 31  Isolation of a small number of crystals confirmed the same unit cell parameters reported for 1.
[Ru(PPh 3 ) 3 (ZnMe) 2 H 3 ][BAr F 4 ] (12). A J. Young's resealable ampule was charged with a C 6 H 6 (5 mL) suspension of 3 (96 mg, 0.05 mmol). After being gently heated to fully dissolve the solid, the resulting red solution was degassed (three freeze−pump−thaw cycles) and H 2 (1 atm) added with vigorous stirring. After 5 min, this gave a pale-yellow solution, which upon treatment with hexane (5 mL) afforded a pale-yellow crystalline sample of 12. This was collected and dried under vacuum. Yield: 76 mg (79%). An alternative route to 12 involved stirring a solid sample of 3 under H 2 (1 atm) for ∼2 h, by which time the sample had changed color from red-orange to offwhite. A 31 P NMR spectrum in C 6 D 6 revealed complete conversion to 12. 1  X-ray Crystallography. Data for 6, 7, and 12 were obtained using an Agilent SuperNova instrument and a Cu Kα radiation source. All experiments were conducted at 150 K, and models refined using SHELXL 58 via the Olex2 59 interface. Refinements were largely straightforward, and only points of note will be detailed herein. First, the phenyl rings based on C9 and C15 were treated for 80:20 disorder in the structure of 6. In 7, the asymmetric unit was seen to contain one molecule of the bimetallic complex and two molecules of benzene. The hydride in the former was located and refined without restraints as were the hydride ligands in 12. Unsurprisingly, the anion in the latter structure required some disorder modeling. In particular, fluorine atoms F4−F6 were treated for three-way disorder in a 0.425:0.425:0.15 ratio, while F22−F24 were modeled to take into account 50:50 disorder. Distances and ADP restraints were employed in disordered regions, to assist convergence.
Computational Details. Density functional theory calculations were performed with Gaussian 16 (revision C.01). 60 Ru, Zn, and P centers were described with the Stuttgart RECPs and associated basis sets, 61 and 6-31G** basis sets were used for all other atoms. 62,63 A set of d orbital polarization functions was also added to P (ζ d = 0.387). 64 Electronic structure analyses were performed on geometries using the heavy atom positions derived from the crystallographic studies with H atom positions optimized with the BP86 functional. 65,66 Details of functional testing on the fully optimized structure of 2 are provided in the Supporting Information. Quantum theory of atoms in molecules (QTAIM) 67 used the AIMALL program. 68 NCI calculations were based on the promolecular densities and used NCIPLOT 69 with visualization via VMD. 70 Natural orbitals for chemical valence (NOCV) analyses 71 were performed using the Amsterdam Modeling Suite (AMS) package. 72 Computed geometries are displayed with ChemCraft, 73 and all geometries are supplied as a separate XYZ file (Supporting Information).
NMR spectra of products from reactions of 2 and 3, Xray structural data and metrics for 6, 7, and 12, and computational details (functional testing, electronic structure analyses, QTAIM, and NOCV) (PDF) Structures and Cartesian coordinates (XYZ)